Acoustic attenuation materials

ABSTRACT

Acoustic attenuation materials are described that comprise outer layers of a stiff material sandwiching a relatively soft elastic material therebetween, with means such as spheres, discs or wire mesh being provided within the elastic material for generating local mechanical resonances that function to absorb sound energy at tunable wavelengths.

FIELD OF THE INVENTION

[0001] This invention relates to novel materials for attenuating sound,and in particular to such materials that are able to attenuate lowfrequency sounds without requiring excessive size or thickness.

BACKGROUND OF THE INVENTION

[0002] The general increase in noise in many environments, both at workand at home, means that noise is becoming a significant source ofpollution, and a factor that can harm both the physical and mentalhealth of many people who are exposed to unwanted noise for prolongedperiods. Noise reduction techniques and materials are therefore becomingof increasing importance.

[0003] Noise reduction can be achieved by either active methods, such aselectronically generated noise cancellation techniques, or by passivetechniques such as simple barriers. Most passive barriers, such as thosemade of fibres or acoustic foam, attenuate the sound by forcing thesound waves to change direction repeatedly. With each change ofdirection a portion of the energy of the sound wave is absorbed (and isin fact converted to heat). Such materials tend to be relativelightweight and are quite effective at attenuating noise at medium andhigher frequencies, such as for example about 500 Hz and above.

[0004] Passive barrier are less effective however, at lower frequencies.A particular problem for example is illustrated by the so-called “masslaw” which requires the thickness of the barrier material to be ininverse proportion to the frequency of the sound. As an example, ittakes five times more mass of material to be an effective barrier at 200Hz than it does at 1000 Hz. A concrete wall, for example, must be about30 cm thick to be an effective barrier at 150 Hz. This increase inthickness and weight means that simple barrier structures are noteffective in practical terms for attenuating low frequency sounds.Attempts to design suitable barrier structures for low frequency soundsinclude, for example, the use of an air-space between two rigid panels.The amount of low-frequency attenuation depends on the spacing betweenthe panel and thus this design again results in a physically largebarrier.

PRIOR ART

[0005] An example of a prior design for a material for acousticattenuation is described in U.S. Pat. No. 5,400,296 (Cushman et al). InCushman et al particles are embedded in a matrix material, the particlesincluding both high and low characteristic acoustic impedance particles.The idea in Cushman et al is that by creating such an impedancemismatch, a portion of the impinging acoustic energy is reflected andthus the energy transmitted is attenuated.

SUMMARY OF THE INVENTION

[0006] According to the present invention there is provided an acousticattenuation material comprising outer layers of a stiff materialsandwiching a relatively soft elastic material therebetween, and whereinmeans are provided within said elastic material for generating localmechanical resonances.

[0007] Preferably the resonance generating means comprises a rigidmaterial located within the elastic material, and the rigid material hasa volume filling ratio within the elastic material of from about 5% to11%.

[0008] One example of a rigid material is a plurality of individualsolid particles located within the elastic material. These solidparticles may be any suitable shape such as spheres or discs.

[0009] Another possibility is that the rigid material may comprise awire mesh. Such a mesh is preferably generally planar and the wire meshlies in the plane of the material. In one embodiment means are providedfor supporting the mesh within the elastic material, for example thematerial may include a surrounding frame member and means may beprovided for securing the mesh to the frame member, such as elasticconnection members.

[0010] In one possibility the rigid material comprises a plurality ofwire mesh segments, and a plurality of frame members may be providedbetween the segments, and wherein means are provided for elasticallyconnecting the segments to the frame members.

[0011] The stiff outer layers may be formed of any suitable buildingmaterial such as gypsum, aluminum, cement, plywood, paperboard, polymermaterials or any other stiff building materials.

[0012] The elastic material may be any relatively soft elastic materialsuch as foam or foam-like materials, natural and synthetic rubber andrubber-like materials, fiberglass, elastic polymer materials and thelike.

[0013] The rigid material may be a metal.

[0014] Viewed from another broad aspect of the invention there isprovided an acoustic attenuation material comprising two outer layers ofa stiff material sandwiching a layer of relatively soft elastic materialtherebetween, and a plurality of solid particles disposed throughoutsaid elastic material.

[0015] The dimensions and material of the particles, and the thicknessand material of the elastic layer, are chosen so as to define aplurality of local mechanical resonances at a frequency to beattenuated. The frequency is preferably in the range of 100 to 200 Hz.

[0016] Viewed from a still further aspect of the invention there isprovided an acoustic attenuation material comprising two outer layers ofa stiff material sandwiching a layer of relatively soft elastic materialtherebetween, and a wire mesh disposed throughout said elastic material.

[0017] The wire mesh is preferably parallel to the outer layers.

[0018] In this embodiment of the invention the dimensions and materialof the mesh, and the thickness and the material of the elastic layer,may be chosen so as to define a plurality of local mechanical resonancesat a frequency to be attenuated.

[0019] Viewed from a still further broad aspect the present inventionprovides a method of forming an acoustic attenuation materialcomprising:

[0020] (a) providing two outer layers of a stiff material sandwiching alayer of an elastic material, and

[0021] (b) providing means within said elastic layer for generatinglocal mechanical resonances at the frequency to be attenuated.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Some embodiments of the invention will now be described by way ofexample and with reference to the accompanying drawings, in which:

[0023]FIG. 1 is a side sectional view through a material according to afirst embodiment of the invention,

[0024]FIG. 2 is a planar sectional view of the material of FIG. 1,

[0025]FIG. 3 is a plot showing the low frequency attenuation ofmaterials according to the present invention in comparison with theprior art,

[0026]FIG. 4 is a plot illustrating the effect on the attenuation ofvarying the particle size,

[0027]FIG. 5 is a plot illustrating the effect on the attenuation ofvarying the material thickness,

[0028]FIG. 6 is a planar sectional view of a material according to asecond embodiment of the invention,

[0029]FIG. 7 is a planar sectional view of a material according to athird embodiment of the invention,

[0030]FIG. 8 is a planar sectional view of a material according to afourth embodiment of the invention,

[0031]FIG. 9 is a plot illustrating the effect on the attenuation ofvarying the shape of the particles, and

[0032] FIGS. 10(a) and (b) are planar sectional views illustratingvariations of the embodiments of FIGS. 6 and 7.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0033] Referring firstly to FIGS. 1 and 2 there is shown a firstembodiment of an acoustic attenuation material according to anembodiment of the invention. In this embodiment an acoustic attenuationmaterial 10 comprises two rigid outer layers 11 sandwiching a softelastic layer 12 within which are located solid particles 13 having arelatively high density and a relatively high rigidity. The particleshave a diameter that is preferably 0.1 mm or larger. As can be seen inFIG. 2, the solid particles 13 are located in a regular grid arrayconfiguration.

[0034] Suitable materials for the rigid outer layers 11 include gypsum,aluminum, cement, plywood, paperboard, rigid polymer materials or anyother conventional rigid building materials. The soft elastic layer 12may be formed of a material such as foam or foam-like materials, naturaland synthetic rubber and rubber-like materials, fiberglass, elasticpolymer materials and the like. The solid particles 13 may be formed ofmetal such as lead, steel, iron or aluminum and aluminum alloys.

[0035]FIG. 3 plots the attenuation against frequency in a low frequencyrange for an embodiment of the present invention formed in accordancewith FIGS. 1 and 2, and with examples of the prior art for reference. InFIG. 3, reference numeral 14 is used to identify the attenuationcharacteristics for an embodiment of the present invention formed of a24 mm thick foam layer 12 in which are located 15 mm diameter lead balls13. The outer rigid layers 11 are formed of two half-inch gypsum boards.The volume filling ratio of the lead balls 13 is 11%. In this embodimentthey are dispersed uniformly throughout the foam layer 12, though thisis not essential.

[0036] As can be seen from FIG. 3, the embodiment of the inventionindicated in that Figure by reference numeral 14 has a strongtransmission loss that peaks at about 175 Hz. In FIG. 3 referencenumeral 15 represents the same structure as this embodiment of theinvention but without the lead balls, 16 is a 24 mm thick cementbarrier, and 17 is an attenuator formed of two half-inch gypsum boardswith a 24 mm air gap therebetween.

[0037] Comparing the four materials 14, 15, 16 and 17 it will be seenthat at higher frequencies, eg above 250 Hz cement 16 is the bestattenuator in terms of performance because it is the most dense. Belowabout 250 Hz the three prior art configurations 15, 16 and 17 are allsignificantly less efficient than the embodiment of the invention 14. Inparticular, at the peak of the absorption of the embodiment of theinvention, an extra 20 dB transmission loss can be obtained using theembodiment of the invention.

[0038] It is believed that the present invention functions by thegeneration of built-in local resonances. By combining high-density solidparticles within a softer foam matrix, a low frequency mechanicalresonance is formed where the solid particles may be regarded as ballsand the softer elastic foam represents a spring. When the frequency ofthe sound approaches the local mechanical resonances and energy istransferred from the impinging sound wave to the balls. Effectivelytherefore there is a band-gap surrounding the absorption peakcorresponding to frequencies that cannot be transmitted through thematerial.

[0039]FIG. 4 shows the same plot as FIG. 3 but with the addition of anew curve 18 that corresponds to another embodiment of the invention.This embodiment is identical to curve 14 but with smaller lead balls 13that are 10 mm in diameter. It can be seen that in this embodiment theattenuation peak is at a slightly higher frequency (approximately 220Hz). This is consistent with the theory because with small balls therewould be local resonances at higher frequencies. As shown in FIG. 5, theattenuation peak may also be varied by changing the thickness of thefoam elastic layer. In FIG. 5 reference numeral 19 refers to an acoustcattenuation materila of the same structure as reference numeral 14 butwith a thickness of the elastic layer of 19 mm. It will be seen that theattenuation peak is shifted to a slightly higher frequency (approx 220Hz).

[0040] In the abovedescribed first embodiment of the invention, thesolid particles are in the form of solid balls arranged, preferably butnot essentially, in a regular grid-like array. In the embodiment of FIG.6 these balls are replaced by a wire mesh 23, for example of iron with a6 mm diameter and a filling ratio of 8.5%. FIG. 7 shows a furtherembodiment in which the wire mesh of FIG. 6 is divided into an array 24of smaller mesh segments still with a wire diameter of 6 mm and afilling ratio of 5.6%. FIG. 8 shows a still further embodiment in whichindividual solid particles are provided, but of a different form fromthe balls of the first embodiment. In the embodiment of FIG. 8 aplurality of disks 25 are provided. These disks, which may be any of thesame materials as the balls, may for example have a diameter of 26 mmand a thickness of 3 mm (filling ratio 5%).

[0041] It will be understood that the attenuation characteristics, suchas the location and width of the attenuation peak, can be varied byappropriately selecting from parameters such as the shape andconfiguration of the particles, their size, filling ratio and material.For example, two or more different sizes of balls may be used to obtainmore than one resonant frequency and thus a broader attenuationresponse. Similarly the size of the discs may be varied and two or moresizes may be provided. Effectively therefore the attenuation response ofthe material of the present invention is “tunable” to provide a desiredattenuation characteristic. FIG. 9 shows the attenuation obtainable withthe wire mesh 23, wire mesh segments 24 and disks 25 as described above.All these embodiments show good attenuation properties at frequenciesbetween 100 and 200 Hz.

[0042]FIG. 10(a) shows an embodiment of the material in which the solidparticles are constrained from “sinking”, ie shifting position, withinthe softer elastic material. In this embodiment, in which the solidmaterial is in the form of a wire mesh 23, the mesh 23 is connected atits edges to a surrounding frame 26 by elastic material such as springs27. Alternatively, as shown in FIG. 10(b), especially when either meshsegments 24 are used or when a large number of individual solidparticles are provided, individual supporting frame members 28 may beprovided within the elastic material.

[0043] The present invention, at least in its preferred forms, provideseffective low-cost acoustic attenuation materials that may be usedeffectively at low frequencies that in the prior art would require largeand heavy acoustic barriers. The attenuation of the material can beselected by appropriate design of the size and shape of the rigidparticles or mesh, the thickness of the elastic layer and the choice ofmaterials. As such the invention can provide materials suitable for awide range of domestic and industrial applications where noisereduction, especially at low frequencies, is required.

1. An acoustic attenuation material comprising outer layers of a stiffmaterial sandwiching a relatively soft elastic material therebetween,and wherein means are provided within said elastic material forgenerating local mechanical resonances.
 2. A material as claimed inclaim 1 wherein said resonance generating means comprises a rigidmaterial located within said elastic material.
 3. A material as claimedin claim 2 wherein said rigid material has a volume filling ratio withinsaid elastic material of from about 5% to 11%.
 4. A material as claimedin claim 2 wherein said resonance generating means comprise a pluralityof individual solid particles located within said elastic material.
 5. Amaterial as claimed in claim 4 wherein said solid particles are spheres.6. A material as claimed in claim 4 wherein said solid particles arediscs.
 7. A material as claimed in claim 2 wherein said rigid materialcomprises a wire mesh.
 8. A material as claimed in claim 7 wherein saidmaterial is generally planar and said wire mesh lies in the plane ofsaid material.
 9. A material as claimed in claim 7 wherein means areprovided for supporting said mesh.
 10. A material as claimed in claim 9wherein said material includes a surrounding frame member and means areprovided for securing said mesh to said frame member.
 11. A material asclaimed in claim 10 wherein said securing means comprise elasticconnection members.
 12. A material as claimed in claim 2 wherein saidrigid material comprises a plurality of wire mesh segments.
 13. Amaterial as claimed in claim 12 wherein a plurality of frame members areprovided between said segments, and wherein means are provided forelastically connecting said segments to said frame members.
 14. Amaterial as claimed in claim 1 wherein said stiff outer layers areformed of gypsum, aluminum, cement, plywood, paperboard, polymermaterials or any other stiff building materials.
 15. A material asclaimed in claim 1 wherein said elastic material is selected from foamor foam-like materials, natural and synthetic rubber and rubber-likematerials, fiberglass, elastic polymer materials and the like.
 16. Amaterial as claimed in claim 2 wherein said rigid material is a metal.17. An acoustic attenuation material comprising two outer layers of astiff material sandwiching a layer of relatively soft elastic materialtherebetween, and a plurality of solid particles disposed throughoutsaid elastic material.
 18. A material as claimed in claim 17 whereinsaid solid particles are spheres.
 19. A material as claimed in claim 17wherein said solid particles are discs.
 20. A material as claimed inclaim 17 wherein the dimensions and material of said particles, and thethickness and material of said elastic layer, are chosen so as to definea plurality of local mechanical resonances at a frequency to beattenuated.
 21. A material as claimed in claim 20 wherein said frequencyis in the range of 100 to 200 Hz.
 22. An acoustic attenuation materialcomprising two outer layers of a stiff material sandwiching a layer ofrelatively soft elastic material therebetween, and a wire mesh disposedthroughout said elastic material.
 23. A material as claimed in claim 22wherein said wire mesh is parallel to said outer layers.
 24. A materialas claimed in claim 22 wherein the dimensions and material of the mesh,and the thickness and the material of the elastic layer, are chosen soas to define a plurality of local mechanical resonances at a frequencyto be attenuated.
 25. A material as claimed in claim 24 wherein saidfrequency is in the range 100 to 200 Hz.
 26. A method of forming anacoustic attenuation material comprising: (c) providing two outer layersof a stiff material sandwiching a layer of an elastic material, and (d)providing means within said elastic layer for generating localmechanical resonances at the frequency to be attenuated.
 27. A method asclaimed in claim 26 wherein said resonance generating means comprises aplurality of solid particles.
 28. A method as claimed in claim 27wherein the attenuation frequency is varied by selecting the dimensions,shape and material of the solid particles, and the thickness andmaterial of the elastic layer.
 29. A method as claimed in claim 26wherein said resonance generating means comprises a wire mesh.
 30. Amethod as claimed in claim 29 wherein the attenuation frequency isvaried by selecting the dimensions, shape and material of the solidparticles, and the thickness and material of the elastic layer.